By now, you’ve probably heard something about how sealing the ductwork in your forced-air heating (and central air conditioning) system can save energy and money. Well, it’s true, no matter how old your house is. Admittedly, this rather tedious job clearly falls in the category of Unsexy Home Energy Improvements (perhaps right below weatherstripping and attic insulation). But unlike high-profile upgrades, such as triple-pane windows and solar panels, sealing ductwork can yield some fairly big savings without an astronomical upfront investment. It’s also easy to do yourself, since it’s pretty much like icing a cake with your hands.

Why Ducts Leak

With apologies to all skilled tinners out there, one of the main reasons ducts leak is because they’re simply (and sometimes poorly) constructed. A standard duct system is made with sheet metal held together with screws, which means it’s not a whole lot different from the gutter system on the outside of your house. Sheet metal is a good material for moving air, but all the joints, fittings and seams leave many opportunities for air leaks to develop. An improperly balanced system compounds this problem by creating unnecessarily high positive and/or negative pressure in the ductwork.

Another cause of big leaks is shoddy workmanship, whether it’s the fault of the original installer, a remodeler or the homeowner. Fittings and seams that aren’t fitted or fastened properly may leak from the beginning or come loose over time. Some of the biggest causes of leaks are repairs or “seals” made with duct tape. You read that correctly. Ordinary duct tape isn’t made for ducts; it dries up and can begin falling off of ductwork within 6 months of application—maybe enough to get you through one heating season, but that’s about it.

For the record, there is “duct tape” made for ductwork, such as UL-181-approved foil tape. This is designed for direct application to metal ductwork. However, mastic is the generally preferred material for sealing most metal ducts (see How to Seal Your Ducts, below).

How Ducts Leak

The simple answer to this question is: Air naturally finds its way out of anything that isn’t airtight. It does this because it’s constantly seeking balance in both pressure and temperature. If warm air is moving through a duct in a cold crawlspace, it’s just waiting for a chance to slip out and mix it up with the cold air outside the duct. To put it another way, air is a master of mingling, better than any socialite at a benefit dinner. Adding pressure to the system—that’s the force in “forced-air”—only makes the air work harder to find a way out.

Where Ducts Leak

A force-air system circulates air through the house via supply and return ducts. Supply ducts bring hot air from the furnace (or cold air from the AC system, which utilizes the furnace) to the heat registers in each room. The return ducts pull cool air from the rooms and bring it back to the furnace for reheating. Therefore, it’s important to seal all of your ducts, not only to prevent the loss of heated air but also to improve air circulation and maintain a proper balance of the system throughout the house.

Ducts can leak at any connection between two parts. Starting at the furnace, leakage can be most pronounced where the main supply duct meets the body of the furnace unit, or air handler. This is where the air pressure is the greatest. Moving down the line, leaks are common where smaller ducts branch off from the main supply duct and at bends and where straight lengths of duct are connected. At the register end, the “boot” fitting that transitions from the duct to the register grill is another likely culprit. All of these same areas are prone to leaking on the return side of the system.

If your ducts are insulated (which is a good thing), don’t let that fool you into thinking sealing isn’t required. Cary Weiner, the Clean Energy Specialist at Colorado State University Extension, warns us that “insulation alone will not stop air leaks. Insulation provides a resistance to heat flow between two spaces but does not stop air leakage. This is where sealing the ducts comes in.”

How to Seal Your Ducts

The best stuff for sealing ducts these days is duct mastic, a water-based adhesive that you simply slop on and spread around with your hands. Once the mastic dries, the seal is complete and shouldn’t have to be tampered with again. For spanning large gaps, it’s a good idea to cover the gap first with self-adhesive, fiberglass-mesh drywall joint tape. This comes in rolls and cuts easily with a sharp utility knife or scissors. You might also need some small sheet metal screws to replace any original screws that are missing or add some where the installer was neglectful.

Before you start, be sure to turn off your furnace. It must remain off until the mastic has cured (check the manufacturer’s directions for curing times). Cary Weiner stresses that “all joints should be sealed, including the connections between the air handler and the ducts, and the seams within the AHU (air handler unit) and the ducts themselves.”

Start at the air handler connections and work your way down the main supply duct and to the end of each branch duct, applying the mastic as directed. Pull away insulation as needed to access joints and other leak-prone areas, and reposition the insulation only after the mastic dries. When the supply ducts are done, repeat the process on all of the return ducts.

Bruce Harley, author of Cut Your Energy Bills Now, offers a great tip for applying mastic: wear vinyl gloves underneath a pair of cotton gloves. This allows you to slip off the cotton gloves and leave them in the mastic whenever you need to switch to a different task.

If you have any questions about what should be sealed and what should not, consult a pro (or at least a knowledgeable neighbor). For example, you should never seal any parts of the air handler itself or the furnace’s flue pipe, which looks like a small duct. You can also have your ducts sealed by a qualified HVAC (heating, ventilation and air conditioning) professional or a duct-sealing company. One professional service uses a special adhesive that seals holes from inside your ducts. This is a good option for sealing any inaccessible ductwork that runs through walls or between floors and finished ceilings.

How Much You Can Save by Sealing Your Ducts

Any energy or HVAC expert will tell you that sealing your ducts should be followed by insulating them, too, at least where they travel through unconditioned (heated or cooled) areas of the home. According to Weiner, “A homeowner can lose up to 30% of the energy used to heat or cool a space if ducts are not properly sealed and insulated, particularly if the ducts run through unconditioned space (such as an attic or basement).”

Weiner adds, “Assuming a homeowner can reduce energy use for space heating by 10 – 25% by sealing ductwork, the average homeowner could save $40 - $100 per year.” It’s important to note that sealing ductwork not only saves energy during the heating season; if you use central AC, it will save you money during the cooling season, too. And thanks to the improved performance of the system, sealed ducts help reduce the workload on your (expensive) heating and cooling plants—that is, your furnace and AC unit. While this isn’t really a measurable factor, it’s worth noting that any mechanical equipment tends to last longer and require less maintenance when it’s working efficiently.

If you really want to know how much sealing your ducts will save, you can have your duct system tested by a home energy auditor or HVAC pro with special pressurizing equipment. Similar to a blower door test, duct testing equipment uses a fan to pressurize the duct network and detects leakage by how much air flow is present. By doing the test before and after you seal your ducts, an operator can calculate how much energy, and therefore money, you’ll save in an average year.